Charge coupled devices (hereinafter also referred to as CCD's) have found wide acceptance as shift registers for transporting charge in image sensors. A series of laterally spaced gate electrodes, adjacent but conductively separate from a semiconductor substrate, are relied upon to transfer the charge in discreet steps within the semiconductor substrate. Each charge transfer step is achieved by proper potential bias in gate electrodes, so that the charge is attracted from one storage region to a next adjacent charge storage region in the semiconductor substrate. Two phase devices are particularly advantageous since they require a minimum of real estate (chip surface area).
An early two phase CCD such as described in Kahng et al, U.S. Pat. No. 3,651,349, includes a semiconductor substrate of a first, N or P conductivity. An insulating layer is formed as a succession of alternating thinner portions and thicker portions overlying the semiconductor substrate, and a series of spaced apart conductive gate electrodes each overlying one thicker and one adjacent thinner portion of the insulating layer.
The art has progressed and shown a preference for two phase CCD's having a simpler insulative layer and gate electrode structure relying on zones of differential impurity doping concentrations in the semiconductor substrate for directionality bias of charge transfer.
Examples of two processes for making two phase CCD's are disclosed in commonly assigned U.S. Pat. Nos. 4,613,402 to Losee et al and 4,746,622 to Hawkins et al.
Heretofore, polysilicon has been the material of choice for gate electrodes. In frame transfer or full frame CCD image sensors, some of the light, more so of shorter wavelength (blue), is absorbed in the polysilicon electrodes and is thus "lost" or not counted as "signal." Transparent gate electrodes, such as indium tin oxide (ITO), tin oxide (SnO.sub.2), or zinc oxide (ZnO), do not absorb as much light as polysilicon electrodes.
Two phase CCDs rely on having two sets of electrodes often formed by first depositing and patterning one set of polysilicon electrodes, then forming an insulating layer over this first set of electrodes and subsequently depositing and patterning a second set of polysilicon electrodes.
The insulating layer between and separating the two sets of electrodes is conveniently formed by converting the surface layer of the first set of electrodes to oxide when the electrodes are made of polysilicon. The prior art does not extend to the use of other materials for forming the first set of electrodes.